This invention relates to optical fibers and, more particularly, to the reduction of dispersion in optical fibers.
In order to transmit information in the form of light pulses on an optical fiber transmission line the transmitted pulses must be individually resolvable at the receiving end of the transmission line. A light ray, however, may take different paths as it traverses the optical fiber. For example, the light ray may proceed directly down to the center of the fiber or it may be reflected off the fiber walls numerous times as it traverses the fiber. Since the distance travelled by a light ray varies on each path, each transmission mode has a different transmission time associated with it. Different paths of a light pulse may traverse the fiber in different modes and thus with different traversal times. As a result, there is a general broadening of the pulse and a consequent loss in pulse resolution.
A method for reducing dispersion in a step-index multimode fiber waveguide is disclosed in U.S. Pat. No. 3,909,110, issued Sept. 30, 1975 to D. Marcuse. As described therein, dispersion is reduced by the introduction of slight fluctuations in the refractive index of the fiber core, which fluctuations deliberately enhance coupling among the various modes in the fiber. Conditions are imposed upon the axial, azimuthal and radial dependence of the core fluctuations. The axial fluctuations take the form of slight perturbations in the refractive index and have a period of approximately 1 mm. A fiber having such small perturbations is both difficult and expensive to fabricate.
Mode dispersion can also be minimized by radially grading the index of refraction of the fiber core from a maximum at the center of the fiber to a minimum at the core-cladding interface. It has been determined that an optimum refractive index profile is parabolic in shape. A fiber with such a smooth continuous profile is not readily fabricated, and is therefore approximated by a plurality of thin cylindrical layers each having a uniform refractive index. The refractive indices of these core layers radially decrease from a maximum at the center of the core to approximate the optimum smooth profile. As can be readily appreciated, as the number of core layers increases, the smooth continuous profile is more closely approximated and the dispersion of a transmitted light impulse decreases. However, in such fibers the theoretical improvement expected from a continuous radial gradation can only be approached, and some pulse broadening still occurs. This pulse broadening increases proportional to the length of the fiber.
A graded index optical fiber such as described hereinabove is readily fabricated using a chemical vapor deposition process described in copending application Ser. No. 444,705, filed Feb. 22, 1974 and assigned to the assignee of the present invention. As described therein, a preform is formed by continuously rotating a silica tube which is traversed by a hot zone. A vapor source material such as the chlorides or hydrides of silicon together with germanium, aluminum, boron, phosphorus, et cetera, and pg,4 oxygen, flows through the tube and reacts in the hot zone to produce glassy "soot" within the vapor and glass on the inner surface of the tube. For each traversal of the hot zone a cylindrical layer of glass is fused into the tube. By varying the composition of the vapor source for each hot zone traversal, a radially graded structure is formed. When the tube is collapsed and a fiber drawn therefrom, the resulting fiber has the same refractive index radial distribution as the preform. As aforenoted, many layers are necessary to approximate the impulse response of a smoothly graded optical fiber. Since the layer is to be fused separately onto the tube, the fabrication time of a preform from which a satisfactory optical fiber can be drawn is long and thus expensive.